On 02/13, Alexei Starovoitov wrote: > On Tue, Feb 12, 2019 at 09:02:32AM -0800, Stanislav Fomichev wrote: > > On 02/05, Stanislav Fomichev wrote: > > > On 02/05, Alexei Starovoitov wrote: > > > > On Tue, Feb 05, 2019 at 07:56:19PM -0800, Stanislav Fomichev wrote: > > > > > On 02/05, Alexei Starovoitov wrote: > > > > > > On Tue, Feb 05, 2019 at 04:59:31PM -0800, Stanislav Fomichev wrote: > > > > > > > On 02/05, Alexei Starovoitov wrote: > > > > > > > > On Tue, Feb 05, 2019 at 12:40:03PM -0800, Stanislav Fomichev > > > > > > > > wrote: > > > > > > > > > On 02/05, Willem de Bruijn wrote: > > > > > > > > > > On Tue, Feb 5, 2019 at 12:57 PM Stanislav Fomichev > > > > > > > > > > <s...@google.com> wrote: > > > > > > > > > > > > > > > > > > > > > > Currently, when eth_get_headlen calls flow dissector, it > > > > > > > > > > > doesn't pass any > > > > > > > > > > > skb. Because we use passed skb to lookup associated > > > > > > > > > > > networking namespace > > > > > > > > > > > to find whether we have a BPF program attached or not, we > > > > > > > > > > > always use > > > > > > > > > > > C-based flow dissector in this case. > > > > > > > > > > > > > > > > > > > > > > The goal of this patch series is to add new networking > > > > > > > > > > > namespace argument > > > > > > > > > > > to the eth_get_headlen and make BPF flow dissector > > > > > > > > > > > programs be able to > > > > > > > > > > > work in the skb-less case. > > > > > > > > > > > > > > > > > > > > > > The series goes like this: > > > > > > > > > > > 1. introduce __init_skb and __init_skb_shinfo; those will > > > > > > > > > > > be used to > > > > > > > > > > > initialize temporary skb > > > > > > > > > > > 2. introduce skb_net which can be used to get networking > > > > > > > > > > > namespace > > > > > > > > > > > associated with an skb > > > > > > > > > > > 3. add new optional network namespace argument to > > > > > > > > > > > __skb_flow_dissect and > > > > > > > > > > > plumb through the callers > > > > > > > > > > > 4. add new __flow_bpf_dissect which constructs temporary > > > > > > > > > > > on-stack skb > > > > > > > > > > > (using __init_skb) and calls BPF flow dissector program > > > > > > > > > > > > > > > > > > > > The main concern I see with this series is this cost of skb > > > > > > > > > > zeroing > > > > > > > > > > for every packet in the device driver receive routine, > > > > > > > > > > *independent* > > > > > > > > > > from the real skb allocation and zeroing which will likely > > > > > > > > > > happen > > > > > > > > > > later. > > > > > > > > > Yes, plus ~200 bytes on the stack for the callers. > > > > > > > > > > > > > > > > > > Not sure how visible this zeroing though, I can probably try > > > > > > > > > to get some > > > > > > > > > numbers from BPF_PROG_TEST_RUN (running current version vs > > > > > > > > > running with > > > > > > > > > on-stack skb). > > > > > > > > > > > > > > > > imo extra 256 byte memset for every packet is non starter. > > > > > > > We can put pre-allocated/initialized skbs without data into > > > > > > > percpu or even > > > > > > > use pcpu_freelist_pop/pcpu_freelist_push to make sure we don't > > > > > > > have to think > > > > > > > about having multiple percpu for irq/softirq/process contexts. > > > > > > > Any concerns with that approach? > > > > > > > Any other possible concerns with the overall series? > > > > > > > > > > > > I'm missing why the whole thing is needed. > > > > > > You're saying: > > > > > > " make BPF flow dissector programs be able to work in the skb-less > > > > > > case". > > > > > > What does it mean specifically? > > > > > > The only non-skb case is XDP. > > > > > > Are you saying you want flow_dissector prog to be run in XDP? > > > > > eth_get_headlen that drivers call on RX path on a chunk of data to > > > > > guesstimate the length of the headers calls flow dissector without an > > > > > skb > > > > > (__skb_flow_dissect was a weird interface where it accepts skb or > > > > > data+len). Right now, there is no way to trigger BPF flow dissector > > > > > for this case (we don't have an skb to get associated > > > > > namespace/etc/etc). > > > > > The patch series tries to fix that to make sure that we always trigger > > > > > BPF program if it's attached to a device's namespace. > > > > > > > > then why not to create flow_dissector prog type that works without skb? > > > > Why do you need to fake an skb? > > > > XDP progs work just fine without it. > > > What's the advantage of having another prog type? In this case we would > > > have > > > to write the same flow dissector program twice: first time against > > > __skb_buff > > > interface, second time against xdp_md. > > > By using fake skb, we make the same flow dissector __sk_buff BPF program > > > work in both contexts without a rewrite to an xdp interface (I don't > > > think users should care whether flow dissector was called form "xdp" vs > > > skb > > > context; and we're sort of stuck with __sk_buff interface already). > > Should I follow up with v2 where I address memset(,,256) for each packet? > > Or you still have some questions/doubts/suggestions regarding the problem > > I'm trying to solve? > > sorry for delay. I'm still thinking what is the path forward here. No worries, thanks for sticking with me :-)
> That 'stuck with __sk_buff' is what bothers me. I might have use the wrong word here. I don't think there is another option to be honest. Using __sk_buff makes flow dissector programs work with fragmented packets; if we were to use xdp_meta instead, it would not work in this case. Another point here: the fact that eth_get_headlen calls flow dissector on a chunk of data instead of skb feel like an implementation detail. Imo, application writers should not care about this context; coding against __sk_buff feels like the best we can do. > It's an indication that api wasn't thought through if first thing > it needs is this fake skb hack. > If bpf_flow.c is a realistic example of such flow dissector prog > it means that real skb fields are accessed. > In particular skb->vlan_proto, skb->protocol. I do manually set skb->protocol to eth->h_proto in my proposal. This is later correctly handled by bpf_flow.c: parse_eth_proto() is called on skb->protocol and we correctly handle bpf_htons(ETH_P_8021Q) there. So existing bpf_flow.c works as expected. Related: I was also thinking about moving this check out of bpf_flow.c and pass n_proto directly. I don't see why bpf_flow_keys "export" n_proto, we know it when we call flow dissector, there is no need to test for skb->vlan_present in the bpf program. > These fields in case of 'fake skb' will not be set, since eth_type_trans() > isn't called yet. Just to reiterate, I do set skb->protocol manually and skb->vlan_preset == false (and bpf_flow.c handles this case). We can also set correct vlan_preset with an additional check, but I decided to not do it since bpf_flow.c handles that. > So either flow_dissector needs a real __sk_buff and all of its fields > should be real or it's a different flow_dissector prog type that > needs ctx->data, ctx->data_end, ctx->flow_keys only. > Either way going with fake skb is incorrect, since bpf_flow.c example > will be broken and for program writers it will be hard to figure why > it's broken. It's fake in a sense that it's stack allocated in this particular case. To the bpf_flow.c it looks like a real __sk_buff. I don't see why bpf_flow.c example might be broken in this case, can you elaborate? The goal of this patch series was to essentially make this skb/no-skb context transparent to the bpf_flow.c (i.e. no changes from the user flow programs). Adding another flow dissector for eth_get_headlen case also seems as a no go.
